Collective modes in electron-hole liquids

Under favourable conditions, a collective mode of the acoustic kind can exist in degenerate electron-hole liquids. The dispersion and damping of such a mode in a model semiconductor is calculated within a generalized random phase approximation and the implications of the results are discussed.     

Shear induced anisotropy in two-dimensional liquids

The behavior of a dense two-dimensional soft disc liquid under shear is studied via nonequilibrium molecular dynamics. The structure factor for the liquid at a given shear rate is evaluated directly by plotting the particle positions, taken at random from the NEMD simulation at that shear, onto photographic film and using light scattering to obtain a diffraction pattern. The pair correlation function of this system is also extracted directly by histogramming the particle positions with respect to a given central particle as a function of separation and angle. The pair correlation function is compared to that approximated by a Fourier series expansion to rank ten. Results are reported as a function of shear rate from a shear rate of 0.1 (when the fluid is essentially Newtonian) to 10 (when the fluid can display a string phase). The appearance of the string phase is discussed and shown to be a consequence of the definition of temperature in the simulation algorithm. A modification of the algorithm is proposed. Comparisons between this work and previous work with three-dimensional liquids are given. The two-dimensional structure factor is compared with that obtained from a real colloidal suspension via light scattering.     

Rotational velocity correlation functions from the pseudo-lattice far infra-red profiles of molecular liquids

The observation of absorption peaks in the far infra-red power absorption profiles of molecular liquids is analysed with the rotational velocity correlation function. These are oscillatory and much longer lived than their equivalents from “conventional” broad band data for molecular liquids in the far infra red.     

Shear banding in mesoscopic dusty plasma liquids

We experimentally demonstrate shear banding and construct a microscopic dynamic picture of a sheared 2D mesoscopic dust Coulomb liquid at the kinetic level. Under the topological constraints from the discreteness and finite boundary, the nonlinear threshold-type response of motion to the local stress induced by thermal and external drives leads to shear thinning and the enhanced avalanche-type local topological transitions with stress relaxation in the form of clusters. It causes the formation of the outer shear bands in which the mean shear rate, the velocity fluctuations, and the structural rearrangement rate are all enhanced, and leaves a weakly perturbed center band. The typical size of the cooperative hopping vortex (about three interparticle distance) sets up a common length scale for the widths of the confinement induced layering and the shear band.     

Shear viscosity and shear thinning in two-dimensional Yukawa liquids

A two-dimensional Yukawa liquid is studied using two different nonequilibrium molecular dynamics simulation methods. Shear viscosity values in the limit of small shear rates are reported for a wide range of Coulomb coupling parameter and screening lengths. At high shear rates it is demonstrated that this liquid exhibits shear thinning; i.e., the viscosity eta diminishes with increasing shear rate. It is expected that two-dimensional dusty plasmas will exhibit this effect.     

Shear magnetoelastic modes in a ferromagnetic cylinder

Features of the mode spectrum of shear magnetoelastic waves circulating along the boundary of a ferromagnetic cylinder are discussed in the nonexchange magnetostatic approximation. Depending on the level of the spin-phonon coupling in the crystal and on the frequency, the lowest order normal mode may be either fast or slow relative to the bulk shear wave in an infinite crystal. It is shown that, unlike the lowest order mode, the higher order modes, which primarily depend on the curvature of the boundary, demonstrate a weak nonreciprocity of propagation in directions with opposite azimuths. Conditions for the slow propagation of the lowest order mode, when it is identified with a surface magnetoelastic wave, are only possible sufficiently far away from the cutoff frequency of the mode spectrum in ferromagnetic cylinders with a strong spin-phonon coupling and a small curvature of the boundary.     

Shock wave structure in simple liquids

The shock wave structure in a liquid is studied by a molecular dynamics simulation method. The interaction between atoms is described by the Lennard-Jones (6–12) potential. In contrast to earlier works, the simulation is performed in a reference frame tied to the shock wave front. This approach reduces non-physical fluctuations and makes it possible to calculate the distribution functions of the kinetic and potential energy for several cross sections within the shock layer. The profiles of flow variables and their fluctuations are found. The surface tension connected with pressure anisotropy within the shock front is calculated. It is shown that the main contribution to the surface tension coefficient comes from the mean virial. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 9, 722–727 (10 May 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

超声波在液体中相速与群速的测量

简要叙述了相速与群速的定义,并用实验测量了超声波在不同液体中的相速与群速.     

Isotope shifts in spectra of molecular liquids

In the IR absorption spectra of low-temperature molecular liquids, we have observed anomalously large isotope shifts of frequencies of vibrational bands that are strong in the dipole absorption. The same effect has also been observed in their Raman spectra. At the same time, in the spectra of cryosolutions, the isotope shifts of the same bands coincide with a high accuracy (±(0.1–0.5) cm^–1) with the shifts that are observed in the spectra of the gas phase. The difference between the spectra of examined low-temperature systems is caused by the occurrence of resonant dipole–dipole interactions between spectrally active identical molecules. The calculation of the band contour in the spectrum of liquid freon that we have performed in this work taking into account the resonant interaction between states of simultaneous transitions in isotopically substituted molecules can explain this effect.     

Rigidity and normal modes in random matrix spectra

We consider the Gaussian ensembles of random matrices and describe the normal modes of the eigenvalue spectrum, i.e., the correlated fluctuations of eigenvalues about their most probable values. The associated normal mode spectrum is linear, and for large matrices, the normal modes are found to be Chebyshev polynomials of the second kind. We contrast this with the behaviour of a sequence of uncorrelated levels, which has a quadratic normal mode spectrum. The difference in the rigidity of random matrix spectra and sequences of uncorrelated levels can be attributed to this difference in the normal mode spectra. We illustrate this by calculating the number variance in the two cases.     

Quasi-normal modes and gravitational wave astronomy

We review the main results obtained in the literature on quasi-normal modes (QNM) of compact stars and black holes, in the light of recent exciting developments of gravitational wave (GW) detectors. QNMs are a fundamental feature of the gravitational signal emitted by compact objects in many astrophysical processes; we will show that their eigenfrequencies encode interesting information on the nature and on the inner structure of the emitting source and we will discuss whether we are ready for a GW asteroseismology.     

Cavitation-induced shock wave behaviour in different liquids

This paper follows our earlier work where a strong high frequency pressure peak has been observed as a consequence of the formation of shock waves due to the collapse of cavitation bubbles in water, excited by an ultrasonic source at 24 kHz. We study here the effects of liquid physical properties on the shock wave characteristics by replacing water as the medium successively with ethanol, glycerol and finally a 1:1 ethanol–water solution. The pressure frequency spectra obtained in our experiments (from more than 1.5 million cavitation collapsing events) show that the expected prominent shockwave pressure peak was barely detected for ethanol and glycerol, particularly at low input powers, but was consistently observed for the 1:1 ethanol–water solution as well as in water, with a slight shift in peak frequency for the solution. We also report two distinct features of shock waves in raising the frequency peak at MHz (inherent) and contributing to the raising of sub-harmonics (periodic). Empirically constructed acoustic pressure maps revealed significantly higher overall pressure amplitudes for the ethanol–water solution than for other liquids. Furthermore, a qualitative analysis revealed that mist-like patterns are developed in ethanol–water solution leading to higher pressures.     

Microscopic theory of the long-wavelength modes of two-component plasmas and ionic liquids: III. The space-time correlation functions

The identity between the exact screening length obtained from the static charge density correlation function and the one which appears in the Einstein relation between the transport coefficients of electrical conductivity and mass diffusion is demonstrated from first principles. For the space-time correlation functions of the number densities we show that their long-wavelength behaviour is completely determined by the four hydrodynamical modes of the two-component system of neutral particles. For charged particle systems there are only three hydrodynamical modes while we have moreover to add the two charge relaxation modes in order to exhaust the long-wavelength limit of the first sum-rule. The strengths with which the various modes appear in the space-time correlation functions have been computed exactly in the limit of long wavelengths.     

Raman spectra of acetonitrile in imidazolium ionic liquids

Raman spectra of dilute solutions of acetonitrile in ionic liquids reveal the characteristic features of ionic liquids' polarity. This is accomplished by investigating the Raman bandshape of the ν (CN) band, corresponding to the CN stretching mode of CH₃CN, which is a very sensitive probe of the local environment. The amphiphilic nature of the CH₃CN molecule allows us to observe the effect of electron pair acceptor and electron pair donor characteristics on ionic liquids. It has been found that the overall polarity of nine different ionic liquids based on 1‐alkyl‐3‐methylimidazolium cations is more dependent on the anion than cation. The observed wavenumber shift of the ν (CN) band of CH₃CN in ionic liquids containing alkylsulfate anions agrees with the significant different values previously measured for the dielectric constant of these ionic liquids. The conclusions obtained from the analysis of the ν (CN) band were corroborated by the analysis of the symmetric ν₁ (CD₃ ) stretching mode of deuterated acetonitrile in different ionic liquids. Copyright © 2010 John Wiley & Sons, Ltd.     

Two-dimensional Yukawa liquids: correlation and dynamics

A combined theoretical and molecular dynamics (MD) simulation study of the collective modes and their dispersion in a two-dimensional Yukawa system in the strongly coupled liquid state is presented. The theoretical analysis relies upon the quasilocalized charge approximation; the MD simulation generates static pair correlation functions and dynamical current-current correlation spectra.     

Causality and the velocity of acoustic signals in bubbly liquids

Several versions of the dispersion formula governing the acoustic propagation in bubbly liquids are shown to exhibit acausal behavior. The cause of this behavior is due to the inappropriate application of a low frequency approximation in the determination of the extinction of the signal from radiative scattering. Using a corrected causal formula, several principles of wave propagation in bubbly media consistent with the general theory of wave propagation in dispersive media are demonstrated: There exist two precursors to any finite signal. Both propagate without regard to the source characteristics at velocities, frequencies, and amplitudes dependent wholly upon the characteristics of the medium supporting the wave motion. The first travels at the infinite frequency phase velocity that is coincident with the infinite frequency limit of the group velocity. That part of a propagating wave oscillating at the source frequency arrives at a time determined by the signal velocity. Analogous to the well known signal velocity of electromagnetic wave propagation in conducting media, the value of the signal velocity depends on the detailed structure of the dispersion formula in the complex frequency plane.     

New resonant absorptions in liquids - local lattice modes

It is shown conclusively that the far infrared spectra of liquids (polar and non-dipolar) are composed of sharp resonant absorptions which may be compared directly with the lattice modes resolved in the solid. This is an unexpected discovery which shows that local molecular structure is preserved on passing from the solid state into the liquid. The observations have wide ranging consequences for condensed state physics which demands we reappraise some of the most basic concepts on which our classical theories of liquid state matter are based.     

Superluminal electromagnetic focus wave modes

A particular transformation of coordinates, associated with superluminal X-pulses, leaves the wave equation invariant and changes focus wave modes into superluminal focus wave pulses. Rather simple and manageable expressions for TM electromagnetic waves allow the investigation of these new localized solutions of Maxwell's equations. Received 11 June 2002 / Received in final form 27 August 2002 Published online 31 December 2002